1. Field of the Invention
The present invention relates to an acceleration sensor and, more particularly, to an acceleration sensor in which fluctuations in the temperature-dependent offset voltage are suppressed and which is able to detect the respective acceleration on three axes.
2. Description of the Related Art
Among acceleration-detection semiconductor acceleration sensors that are used in automobiles, ships, toys, and portable terminals, and so forth, acceleration sensors that utilize the piezoresistive effect, the piezoelectric effect, and variations in the electrostatic capacity, and so forth, and which use a variety of detection means have been developed. These acceleration sensors must be miniature and have a high performance.
In addition, the development of sensors that permit multiple-axis detection by means of one chip has progressed and such sensors have already been produced. Among such sensors, the development of multiple-axis acceleration sensors of the piezoresistive type that employ a semiconductor substrate and which can be manufactured by using general semiconductor technology has advanced.
For a general acceleration sensor, a beam structure in which an weight and a frame portion that is disposed to enclose the weight are supported by a plurality of beams that link the frame and weight or a diaphragm structure in which the weight is supported by a thin film are typical. Further, detection in a case where acceleration is applied involves detecting bending that is produced in the beams or diaphragm by means of detection elements.
In a case where piezoresistive elements are formed on the beams or diaphragm as detection elements and acceleration is applied, the weight swings vertically or laterally and stress acts on the beams or diaphragm that support the weight. The acceleration can be grasped as the variation in the resistance of the piezoresistive element in accordance with the bending of the beam caused by the stress.
FIG. 1 is a conceptual view to illustrate a structure of a sensing portion constituting the main element of the acceleration sensor. FIG. 1A is a perspective view of the sensing portion; FIG. 1B is a planar view of the sensing portion, and FIG. 1C is a cross-sectional view of a state where the sensing portion is mounted on a glass substrate.
In FIGS. 1A and 1B, an SOI substrate 10, which is a substrate for fabricating the sensing portion of the acceleration sensor, is constituted comprising a frame 14, piezoresistive elements 11 that are produced on the SOI substrate 10 by means of a process described subsequently, an weight 12 constituting a movable portion of the sensing portion, and a beam 13 that joins the weight 12 and frame portion 14 and supports the movement of the weight 12. In addition, as shown in FIG. 1C, a glass substrate 15 is fixed to the frame 14 to support the beam 13 and hold the weight 12 so as to face one end face in the axial direction of the weight 12.
In this structure, when the weight 12 constituting the movable portion moves, the movement results in a swing or bending of the beam 13 and, hence, there is a variation in the resistance value of the piezoresistive elements 11 that are provided on the beam 13. The variation in the resistance value is detected as an electrical signal output by using a Wheatstone bridge circuit.
Here, in the case of constitution that employs piezoresistive elements as detection elements as shown in FIG. 1, bending of the beam 13 is grasped as a variation in the resistance of the piezoresistive elements and it is necessary to provide draw wiring (leads) that join an external detection circuit and the piezoresistive elements in order to detect the variation in the resistance of the piezoresistive elements.
FIG. 2 is an enlargement of the planar view of FIG. 1B which shows the connected relationship of the piezoresistive elements and the wiring leads as a general constitution.
In FIG. 2, piezoresistive elements are formed and arranged as detection elements on each of four beams 13a to 13d. 
In the embodiment shown in FIG. 2, piezoresistive elements 11a1 and 11a2 are formed on a beam 13a, piezoresistive elements 11b1 to 11b4 are formed on a beam 13b, piezoresistive elements 11c1 and 11c2 are formed on a beam 13c, and piezoresistive elements 11d1 to 11d4 are formed on a beam 13d. 
External detection portions that are joined via pads 16 to leads connected to the piezoresistive elements 11a1 and 11a2 on the beam 13a and to the piezoresistive elements 11c1 and 11c2 on the beam 13c are connected to a bridge circuit and acceleration in the Y-axis direction is sensed. Similarly, external detection portions that are joined via pads 16 to wiring leads connected to the piezoresistive elements 11b1 and 11b2 on the beam 13b and to the piezoresistive elements 11d3 and 11d4 on the beam 13d are connected to a bridge circuit and acceleration in the X-axis direction is sensed. Further, external detection portions that are joined via pads 16 to wiring leads connected to the piezoresistive elements 11b3 and 11b4 on the beam 13b and to the piezoresistive elements 11d1 and 11d2 on the beam 13d are connected to a bridge circuit and acceleration in the Z-axis direction is sensed.
Here, the problem with an acceleration sensor of the kind described above is the existence of an offset value arising from the temperature characteristic.
That is, a variety of factors may be cited as primary factors arising from the temperature characteristic, namely, variations in the temperature characteristic and resistance of the piezoresistive elements, the internal stress of the wiring, thermal stress, the semiconductor substrate forming the element, differences in the thermal expansion coefficient of the glass substrate or the like that is connected to the semiconductor substrate by anode bonding or similar.
In particular, in cases where leads (wiring) joining the detection elements (piezoresistive elements) 11 and pads 16 do not possess symmetry about the axis of detection (X-axis direction or Y-axis direction) or about the detection elements (piezoresistive elements), the film stress (distribution) caused by the wiring is different for the beams 13a to 13d. 
FIG. 3 shows an enlargement of the part of the beam 13a in FIG. 2 to permit an understanding of this aspect. FIG. 4 shows an enlargement of the part of the beam 13b in FIG. 2.
In FIG. 3, supposing that the piezoresistive elements 11a1 and 11b1 that are formed on the beam 13a constitute the center, leads (wiring) disposed on both sides are formed asymmetrically. Meanwhile, in FIG. 4, the leads (wiring) disposed on both sides are formed asymmetrically between the piezoresistive elements 11b1, 11b3, and 11b2 and 11b4 that are formed on the beam 13b. 
The film stress balance differs minutely in the vicinity of each piezoresistive element as a result of such an asymmetric target arrangement of the wiring leads on the beams 13a and 13b and there is also a shift in the resistance value. Therefore, the resistance balance of the bridge circuit collapses as a result and the offset voltage increases. Further, the offset voltage similarly increases also with respect to the thermal stress of pure aluminum Al, Al alloy (Al—Si, Al—Cu, or the like) that is used for the wiring.
For this reason, a variety of proposals have been made in order to resolve this problem.
As a first example, in the case of the invention that is shown in Japanese Patent Application Laid Open No. 2003-92413, dummy wiring is formed in addition to the wiring resistors and a wiring pattern on the beam is made symmetrical in the direction of the detection axis and in a direction that is perpendicular to the detection axis.
As a second example, the invention shown in Japanese Patent Application Laid Open No. 2003-279592 adopts a method for separating the placement positions of the piezoresistive elements that are arranged on the beams from the point at which there is a concentration of stress for detecting acceleration in the Z-axis direction. This method adjusts the output by enlarging or reducing the interval between two piezoresistive elements arranged on one beam in the axial direction of the beam.
Further, as a third example, in Japanese Patent Application Laid Open No. H11-311631, the effect on the temperature characteristic as a result of differences in the thermal expansion coefficient between the Si substrate and the anode-bonded glass substrate is a problem from another perspective and, in order to reduce this effect, a groove is formed in the glass substrate to absorb or alleviate the stress that is produced as a result of differences in the thermal expansion coefficient.
As a result of the method that provides the dummy wiring and the method that separates the positions of the piezoresistive elements from the point at which there is a concentration of stress that appear in Japanese Patent Application Laid Open Nos. 2003-92413 and 2003-279592 respectively, there is no increase in the steps when dummy wiring is provided as long as a material that is different from that used for the wiring is not used. Therefore, there is no deterioration in comparison with conventional steps from a perspective of producibility or cost.
However, when the dummy wiring that appears in Japanese Patent Application Laid Open No. 2003-92413 is provided, same must be placed by considering the symmetry and, when the width dimension of the beam is determined, for example, a space equivalent to the interval between the width of the dummy wiring and draw wire (wiring lead) is required and it is difficult to narrow the beam width.
Further, also with regards to the shift in the positions of the piezoresistors that appears in Japanese Patent Application Laid Open No. 2003-279592, when the interval between the piezoresistive elements is narrowed, it is hard to shorten the beam length when the shape of the piezoresistive element is not changed. Conversely, it is possible to extend the beam length but, an excessively long beam length produces large fabrication inconsistencies, which is undesirable. In addition, when the beam length is changed, the amount of deformation with respect to the film stress of the beam and so forth also changes. There is then a need to change the thickness of the film stress and the film deposition method (conditions) and so forth.
In addition, when the interval of the piezoresistive element is enlarged by shifting the piezoresistive element 11 in a direction toward the frame 14 or weight 12 (a shift in an outward direction with respect to the center of the beam), the shifting of the wiring on the frame 14 toward the outside as is to the extent of the shift of the piezoresistive element brings about a change in the wiring layout of the frame. Thereupon, when the frame size is fixed, the space for running the wiring is limited and a variation in the wiring is problematic. Further, the fact that the space in which the wiring can be run is limited means that it is hard to reduce the frame, which is also problematic from a miniaturization perspective.
Further, in the case of a method that forms a groove for alleviating stress in the element fabricated by means of the semiconductor substrate or in the glass substrate according to Japanese Patent Application Laid Open H11-311631, increased costs are unavoidable because a step of forming a groove in the anode-bonding glass substrate is added.